Fermentation Basics: Salt, Temperature, and Air

Fermentation is rigging a competition

Every ingredient carries a mixed population of microbes. Fermentation makes conditions favour the lactic acid bacteria so they crowd the rest out. They convert sugar to lactic acid, the pH falls, and spoilage organisms cannot survive a low pH.

So the safety of a ferment does not come from cleanliness — it comes from the acidity dropping fast. Which is why the first few days matter most.

Lever one: salt concentration

Salt sets a level lactic bacteria tolerate and most spoilage organisms do not.

• 2–3% — the usual range for Kimchi and its relatives. Ferments fast, gets properly sour.
• 5%+ — for long storage. Slow, and noticeably salty.
• Under 2% — things go soft and the risk of the wrong microbes winning rises sharply.

Salting the cabbage is the step where you set this concentration. How much you rinse afterwards changes the final figure, so taste a piece after rinsing.

Lever two: temperature

Temperature sets the speed — and shifts the flavour with it.

• 20–25°C — sour within two or three days. It is why kimchi ferments overnight in summer.
• 10–15°C — one to two weeks, with sourness and savouriness developing in balance. The best window.
• Below 4°C — essentially stopped. This is where you hold a ferment you like.

So the method is always the same: ferment at room temperature to the point you want, then move it to the fridge. A kimchi fridge exists to hold that middle band.

Lever three: keeping air out

Lactic bacteria do not need oxygen; moulds do. So pressing everything under the liquid is the whole anti-mould strategy.

• Pack it down so the brine covers the solids.
• Cover the surface with an outer leaf or a sheet of film to limit air contact.
• Do not fill the jar — leave about 20% headspace, because gas is produced. A sealed jar filled to the brim overflows.

Normal versus failed

• A thin white film on the surface → kefir/kahm yeast. Skim it. If it smells sour, it is fine.
• Fuzzy black, green or pink mould → discard it. Scraping the visible part off does not help; the hyphae run deeper.
• Bubbling and a sour smell → normal. It is working.
• Slimy, with a rotten smell → failed. Not enough salt, or too warm.
• Brine gone thick and stringy → wrong microbes. Discard.

Ferments as ingredients

Fermenting is cooking in itself, but most often it is an ingredient for something else.

• Kimchi is three different ingredients: fresh, ripe, and very sour. Very sour kimchi suits Kimchi Bokkeumbap and kimchi stew because the acid carries fat and meat.
• The doenjang in Doenjang Jjigae and the miso in Miso Soup are soybeans fermented with mould and yeast, and boiling kills both the aroma and the live cultures. That is why they go in at the end and are not boiled hard.
• Soy sauce, fish sauce and vinegar are all ferments too. More on that in Building umami.

The one to start with

The hardest ferment to get wrong is brined vegetables. One litre of water, 20–25g salt, radish or cucumber or cabbage held under the surface, two or three days at room temperature. When it bubbles and tastes sour, refrigerate. That single batch teaches all three levers.